This proposal seeks continuation funding for an ongoing research program dedicated to understanding autonomic cardiovascular regulatory mechanisms in man. Past research has centered upon new, noninvasive techniques for altering carotid baroreceptor afferent traffic in precise, reproducible, and highly controllable ways. In these studies, efferent vagal and sympathetic outputs were gauged indirectly from measurements of P-P intervals of the electrocardiogram and intraarterial pressure. In the continuation years, a new technique, percutaneous sympathetic microneurography, will be added. This method permits measurements of efferent sympathetic traffic to be made directly from mixed peripheral nerves of alert, cooperative human volunteers. I expect to realize several scientific goals with these studies. First, I hope to delineate important central autonomic mechanisms by learning how one physiological perturbation, respiration, influences autonomic outflow. Second, I hope to define the precise afferent baroreceptor input profile necessary to inhibit muscle sympathetic outflow, and in so doing, I hope to delineate the central physiology which controls resting plasma norepinephrine concentrations in man. Third, I will capitalize upon the presence of a rare population in this medical center, patients with cardiac transplants, to further define the important role of cardiopulmonary receptors in neural cardiovascular regulation.